This invention relates to permselective barriers in the form of thin membranes useful for the selective separation of mixtures of fluids, fluids and particulates, and solutions. More particularly, this invention relates to a method for fabrication of a permselective membrane suitable for reverse osmosis, nanofiltration, ultrafiltration and the like.
It is well known that dissolved substances, such as salts, minerals, and the like, can be separated from their solvents, such as water, by a technique known as reverse osmosis. For example, the mineral and salt content of seawater can be reduced substantially by utilizing reverse osmosis membranes to produce potable and/or commercially usable low salt water. Similarly, softened water for household or industrial use may be obtained from relatively hard water with high total dissolved solids content. Softened water is important for prolonging the life of various kinds of delicate machinery and for producing water which is usable in a variety of commercial and domestic applications. Perhaps the greatest impact of filtration technology to the increasingly industrialized world is the desalinization of brackish water or seawater to provide large quantities of relatively salt-free water for industrial, agricultural, or residential use. As the world continues to industrialize and population continues to increase at a rapid rate, increasing demands will be made on supplies of fresh water so that availability of efficient and effective mechanisms to convert brackish water and saltwater to productive uses will be increasingly important.
Of importance similar to reverse osmosis membranes are nanofiltration and ultrafiltration membranes, which are used to filter molecules and particulates from solutions and mixtures. For explanation purposes, the discussion below focuses upon reverse osmosis membranes, although one skilled in the art would realize that similar considerations apply to nanofiltration and ultrafiltration membranes. Reference herein to filtration membranes thus includes reverse osmosis, nanofiltration, and ultrafiltration membranes.
Reverse osmosis basically is a filtering out of dissolved ions or molecules by applying a hydraulic pressure to the water to be filtered to force it through a reverse osmosis membrane. Such membranes are selectively permeable for the water, but reject passage of unwanted constituents, typically salts and various dissolved minerals. Osmotic pressure, the tendency of solute components to spread evenly to both sides of a membrane, works against the reverse osmosis process. The more the feed water is concentrated with unwanted components, the greater is the osmotic pressure which must be overcome. Thus, to be practical, a reverse osmosis membrane must strongly reject passage of the unwanted components, commonly referred to in the art as having high rejection characteristics.
In osmosis, the application of pressure to a saline solution that causes movement of water through a solid or liquid barrier while preventing the phases from remixing rapidly requires a theoretical osmotic pressure of 115 psi to desalt a 1% NaCl solution at 25.degree. C. Therefore, the more restrictive the solid or liquid barrier is to solution flow, the higher the pressure required to drive the desalting process. The term associated with restrictive flow is pressure drop. It is intuitive that the thicker the barrier layer existing between the salt and desalted solutions, the higher the pressure required to desalt the solution.
In many instances, reverse osmosis membranes currently are fabricated utilizing a porous substrate upon which is coated a monomer or polymer which is subsequently cross-linked, such as is disclosed in U.S. Pat. No. 4,277,344, issued to Cadotte, which is hereby incorporated by reference. The Cadotte patent reveals that good salt rejection and flow characteristics can be obtained with a reverse osmosis membrane made from cross-linked, interfacially polymerized aromatic polyamides. The membranes created with the Cadotte process represent a n important advancement over prior art membranes due to significant improvements in the ion rejection characteristics, flow characteristics, and resistance to oxidative attack.
The process for making reverse osmosis membranes utilizing Cadotte technology is to coat a porous support layer with a polyamine component. The porous support layer with the polyamine coating is contacted with polyacyl halide, initiating an interfacial polymerization in situ on the support. The resulting product is dried to provide a composite membrane comprising the polyamide laminated to the porous support. The in situ cross-linking provides a mechanical adhesion of the resulting cross-linked reverse osmosis membrane to the support substrate.
Existing methods for performing a coating process like the Cadotte process utilize dip coating technology. In dip coating, the desired substrate is run on a continuous basis and is conveyed through a first liquid bath, coating both sides of the substrate with a first component, e.g., polyamine, and next is conveyed through a second liquid bath containing the second component, e.g., polyacyl halide, after which the membrane is conveyed through any desired rinsing processes, and then through a drying oven. The membrane layers thus formed are typically on the order of from about 0.5 microns to about 1.0 microns.
A primary drawback to the dip coating approach is that the results of the substrate coating process are dependent upon many hard to regulate factors, including the viscosity, cohesion, and adhesion properties of the coatings in the tanks, which properties vary with temperature, solution makeup, and other similar factors. For example, dip tanks depend upon gravity, whereby excess coating is allowed to run off from the substrate. Thus, due to the various factors affecting viscosity, adhesion, and cohesion of the coating, the thickness of the coating is difficult to control. The variations in thickness of the coating applied to the substrate can cause breaks or voids in the membrane coating, resulting in substantially reduced membrane effectiveness or failure of the product involved. It is common in the industry that up to twenty percent of the dip-coated substrate becomes scrap material due to such deficiencies in the resultant coated membrane products.
Further, a dip coating process may result in cross-contamination between the tanks as the substrate is passed from one tank to the next. A certain excess amount of the first constituent which does not sufficiently run off of the substrate is carried into the second tank, causing buildup of the constituents from the first dip tank in the second dip tank. This results in variations in the concentration and make-up of the constituents in the second tank, which variation progressively changes as the process continues, ultimately leading to variations in the effectiveness of the cross-linking occurring between the two coated layers, with corresponding variations in the final membrane. Often, these excess constituents remaining on the substrate must be extracted in subsequent baths of citric acid, bleach, and the like.
Further, the second dip tank is typically sized to be large enough so that an entire batch of membranes (e.g. an eight hour run of material) may be run before the constituent in the second tank becomes so contaminated that it must be discarded. As a result, large amounts of waste dip coating chemicals typically are created in the dip coating process.
In addition, the constituents tend to permeate the porous substrate during the dip coating process, creating several potential problems. First, the thickness of the filter layer of the filtration membrane inversely impacts the capacity of the membrane, whereby the thicker that the filter layer is, the lower is the membrane capacity. Because dip coating constituents tend to permeate the substrate, the resulting filter layer effectively extends into the substrate. In contrast, an ideal filter membrane would have only a thin filter layer formed upon the surface of the substrate. Second, because the dip coating components permeate the substrate, excess components tend to be left in the substrate after the final dip coating operation. These excess components must be cleaned from the membrane by rinsing the coated substrate in a strong solvent such as citric acid. Such rinsing adds to the cost and environmental impact of such processes. Also, due to the process whereby the substrate is dipped into a tank, both sides of the substrate necessarily become coated with the constituents. This resultant back side "quasi membrane" acts to further thicken the membrane, with the associated adverse effects discussed above, and in addition interferes with adhesion requirements for leaf formation when used for the spiral winding process in spiral wound filtration elements.
Due to the variations in thickness inherent in dip coating technology, a relatively thick coating must be applied in order to minimize the number of voids and breakthroughs in the membrane, and is typically on the order of from about 0.5 microns to about 1.0 microns. This thicker coating effectively limits the throughput of the membrane by producing a more substantial barrier to propagation of the water through the membrane, thus reducing the overall functional usefulness of the membrane thus produced. For example, reverse osmosis membranes produced utilizing conventional dip coating techniques typically have a coating which is about 0.5 microns thick or greater, resulting in a nominal 24 gallons per square foot per day (gfd) (40.8 liters per square meter per hour (lmh)) throughput of common seawater with a 98.5% rejection factor.
In order to control some of the problems inherent in dip tank coating of membranes, the substrate is conveyed through the dip tanks at a relatively slow rate, typically about 3 meters (10 feet) per minute. This slow production rate reduces, but does not eliminate, the carryover, thickness variation, and similar problems inherent in dip coating. However, the slow production rate significantly increases the capital cost of the equipment required for a given production rate.
Another limitation of dip tank technology is that significant environmental problems are presented. The chemicals utilized in the tanks typically are quite volatile and potentially harmful to the environment and personnel working in the area. As dip tanks are open to the environment to the extent necessary to allow the film membrane to pass in and out of the liquid interface, they inherently have large surfaces which are required to be sealed from the environment. Therefore, significant environmental limitations on dip coating technology reduce the efficient production of high quality reverse osmosis membranes. Also, allowing some chemical constituents to be relatively open to the environment poses problems in situations in which the constituents are unstable and tend to oxidize or otherwise react due to such exposure to light and air.
Conventional filtration membranes also have been produced by other than dip coating techniques. Ultrafiltration membranes have been produced utilizing coating methods generally referred to as "extrusion" or "knife-over-roll" processes such as are disclosed in U.S. Pat. No. 5,522,991, issued to Tuccelli et al. Coatings utilizing knife-over-roll or extrusion-type processes utilize relatively high viscosity coating fluids and typically have a substantially thicker ultimate coating than is desired for reverse osmosis membranes. For example, in the Tuccelli patent, a coating solution of 8% to 25% solvated N-methyl pyrrolidone (NMP) or dimethyl acetamide (DMAC) is used, which has a viscosity of between about 10,000 centipoise (cps) and about 60,000 cps. In the knife-over-roll or extrusion processes, because of the use of relatively high viscosity fluid, the movement of the membrane past the roller knife or extruder nozzle pulls a portion of the fluid out of the extruder or under the knife. Such a process is not suitable for use on filtration membranes formed utilizing chemistries similar to Cadotte, because such chemistries require a "wet-on-wet" process in which a second wet layer is applied while the previously applied first layer is still wet. Application of a second layer by an extruder or knife-over-roll process would have a tendency to disrupt the uniformity of the previously coated wet layer because of the tension applied to it by the high viscosity of the second fluid being pulled onto the substrate as it passes by the second extruder nozzle or over the knife roller. Thus, the pulling action effectively prevents use of knife-over-roller or extrusion processes for wet-on-wet applications. Further, the film thickness achievable by knife-over-roller and extrusion processes similar to those disclosed in the Tuccelli patent are typically from 5 to 15 microns, with an accuracy of plus or minus approximately 10% of the desired thickness. That film thickness is greater than desired for a reverse osmosis membrane. Thus, it is apparent that neither an extrusion nor a knife-over-roller process is applicable to the production of reverse osmosis membranes utilizing the chemistries described herein.
Slot coating is an established technology which originated in the photographic industry as a high quality process by which multiple micron-thick layers can reliably be coated at relatively high speeds, such as is disclosed in U.S. Pat. No. 5,143,758 issued to Devine. A type of slot coating is also used in coating substrates in the electronics industry such as is disclosed in U.S. Pat. No. 5,516,545 issued to Sandock. Slot coating is a continuous coating technique which delivers quantitatively precise amounts of material, typically of low solids and viscosity characteristics, to an applicator which then deposits quantitatively precise amounts of the material on to a traveling web or other substrate through a slot in the applicator from which the fluid exits. Slot coating technology typically allows web speed s in excess of 100 ft/min (30 meters/min ). As a point of comparison, the dip coating technology utilized in current reverse osmosis membrane coating techniques allows a speed o f substrate movement through the tanks on the order of approximately 10 ft/min (3 meters/min).
Conventional usages of slot coating technologies have generally been limited to smooth nonporous surfaces, such as photographic films and papers and circuit boards, and have further been limited to non-interactive chemistries. For example, in the electronics industry, slot coating typically has been used to apply photoresist and similar single-layer coatings to nonporous substrates, such as individual circuit boards. Typical slot coating of photographic films involves application of multiple layers of liquids which are not interactive with each other or with the substrate. Indeed, typical photographic applications which are slot coated and require multiple wet-on-wet layers use a "cascade" or "slide" coater which applies multiple but often different liquid layers simultaneously from a single coating head having multiple slots, such as is shown in the Devine patent. For each slot in a cascade coater there is a corresponding channel which feeds the desired fluid to the slot orifice. As the fluid flows out of the many slots, the bottom fluid comes out of the die and onto the substrate with the other layers on top of it. There is no intermixing, reaction, or interaction which occurs, as the coating uniformity is carefully controlled via appropriate viscosities and pH levels. Thus, when coating with layers of solutions which are highly reactive in nature, the use of cascade slot coating would not be operative, as the instantaneous formation of polymer would result in coating defects which ultimately translate into poor product performance. In contrast, as described above, creation of filtration membranes typically involves application of interactive low viscosity chemical layers to porous substrates. Application of such multiple layers of interactive chemicals is generally not feasible in a cascade coater, because of cross-contamination of the chemicals and buildup of cross-reacted chemicals on the slot coaters. In addition, the liquid constituents deposited by a cascade coater must not only be n on-reactive, but also must have relatively high viscosity, along the order of 400 cps. Further, slot coating multiple layers using a conventional cascade coater requires that the fluids be aqueous, a s solvent systems can not be used with such a system.
An other technology which has found utilization in the printing industry is gravure coating. Gravure coating is a process by which a thin liquid film is quantitatively deposited onto a substrate by means of a rotating roll. The surface of the gravure roll possesses an engraved pattern of cells, the dimensions of which can be carefully controlled by acid etching or electromechanical techniques. The wet coating thickness is then determined by the cell volume and uniformity. One of the drawbacks to gravure coating is that moderate changes in coating thickness typically require that the gravure roll cell characteristics, i.e., cell depth, pitch, cell wall dimensions, need to be redesigned and subsequently etched in. This is a timely and costly process.